The present invention relates to fuser apparatus for electrostatographic printing machines and in particular to release agent management (RAM) systems for a heat and pressure roll fuser.
In imaging systems commonly used today, a charge retentive surface is typically charged to a uniform potential and thereafter exposed to a light source to thereby selectively discharge the charge retentive surface to form a latent electrostatic image thereon. The image may comprise either the discharged portions or the charged portions of the charge retentive surface. The light source may comprise any well known device such as a light lens scanning system or a laser beam. Subsequently, the electrostatic latent image on the charge retentive surface is rendered visible by developing the image with developer powder referred to in the art as toner. The most common development systems employ developer which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development, the toner particles are attracted from the carrier particles by the charged pattern of the image areas of the charge retentive surface to form a powder image thereon. This toner image may be subsequently transferred to a support surface such as plain paper to which it may be permanently affixed by heating or by the application of pressure or a combination of both.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an adhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon. In these fusing systems, however, since the toner image is tackified by heat it frequently happens that a part of the image carried on the supporting substrate will be retrained by the heated fuser roller and not penetrate into the substrate surface. The tackified toner may stick to the surface of the fuser roll and offset to a subsequent sheet of support substrate or offset to the pressure roll when there is no sheet passing through a fuser nip resulting in contamination of the pressure roll with subsequent offset of toner from the pressure roll to the image substrate.
To obviate the foregoing toner offset problem it has been common practice to utilize toner release agents such as silicone oil, in particular, polydimethyl silicone oil, which is applied to the fuser roll surface to a thickness of the order of about 1 micron to act as a toner release material. These materials possess a relatively low surface energy and have been found to be materials that are suitable for use in the heated fuser roll environment. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner image carried on the support material. Thus, a low surface energy, easily parted layer is presented to the toners that pass through the fuser nip and thereby prevents toner from adhering to the fuser roll surface.
Various systems have been used to deliver release agent fluid to the fuser roll including the use of oil soaked rolls and wicks with and without supply sumps as well as oil impregnated webs. A another type of RAM system is disclosed in U.S. Pat. No. 4,214,549 granted to Rabin Moser on Jul. 29, 1980. As disclosed therein, release agent material is contained in a sump from which it is dispensed using a metering roll and a donor roll, the former of which contacts the release agent material and the latter of which contacts the surface of the heated fuser roll.
U.S. Pat. No. 4,593,992 issued on Jun. 10, 1986 discloses an image forming apparatus for forming an unfixed image on a recording material including a fixing device having a pair of rotatable members for holding therebetween and conveying the recording material to fix the unfixed image on the recording material, speed control device for variably controlling the fixing rotational speed of the pair of rotatable members to a first fixing speed and a second fixing speed lower than the first fixing speed, application apparatus for intermittently supplying a parting agent to at least one of the pair of rotatable members, and application control apparatus for variably controlling the application acting period of the application apparatus in accordance with the fixing rotational speed of the pair of rotatable members variably set by the speed control device.
U.S. Pat. No. 4,496,234 issued on Jan. 29, 1985 discloses a release agent management (RAM) system for use with a heat and pressure fuser. The system is characterized by the use of a simple reciprocating, positive displacement pump for delivering silicone oil to the heated roll of the fuser. The pump is actuated in response to the fuser rolls being engaged and disengaged, such movement being adapted to act against one or the other of a pair of springs which in cooperation with the oil being pumped forms a damper system which is utilized to control the quantity of oil delivered. The springs and oil cause the velocity of the pump's piston to decay with time which results in more oil being pumped initially.
U.S. Pat. No. 4,047,885 issued on Sep. 13, 1977 discloses contact fuser assembly for use in an electrostatic reproducing apparatus including an internally heated metal core cooperating with a resilient backup roll to form a nip through which substrates carrying toner images are moved with the toner images contacting the metal core. The fuser assembly is characterized by the provision of a sump of liquid release agent material which material is provided for coating the surface of the fuser roll structure. In order to apply the liquid release agent material to the surface of the fuser roll structure there is provided a cylindrical applicator member which is partially submersed in the release agent material. A ratchet wheel and pawl arrangement is provided for periodically indexing or moving the applicator member in response to disengagement of the backup roll from the fuser roll through pivoting of an arm supporting the backup roll. To this end, the pawl member is pivotably supported by the pivot arm and actuates the ratchet wheel each time the pivot arm is moved for effecting disengagement of the backup roll from the fuser roll.
A common problem for many xerographic subsystems is meeting the levelness specification for the machine; typically plus/minus 1.5 degrees. In the fusing subsystem the problem is proper performance of a donor roll ram system when "all" of the oil runs to one end of the mini-sump. In this situation, one end of the metering roll will receive little or no oil while the other end receives too much oil.
A solution for some machines has been to move the outlet of the mini-sump from one end of the sump to a position halfway across from the inboard end to the outboard end (ib/ob) of the mini-sump. This basically reduces the variation in oil height from ib to ob to one half the height variation with the inlet on one end and the outlet on the other. A solution such as this presents problems of positioning feed hoses externally of the sump at the middle thereof. This is because feed hoses at this location have the potential of interfering with the movement of the image substrate such as plain paper.